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1.4 Testing Equality of Two Strings

NN 2, IE 3

1.4.1 Problem

You want to compare a user's text entry against a known string value.

1.4.2 Solution

Convert the user input to either all uppercase or all lowercase characters, and then use the JavaScript equality operator to make the comparison:

if (document.myForm.myTextBox.value.toLowerCase( ) =  = "new york") {
    // process correct entry

By using the results of the case conversion method as one of the operands of the equality expression, you do not modify the original contents of the text box. (See Recipe 1.3 if you want to convert the text in the text box to all of one case.)

1.4.3 Discussion

JavaScript has two types of equality operators. The fully backward-compatible, standard equality operator (= =) employs data type conversion in some cases when the operands on either side are not of the same data type. Consider the following variable assignments:

var stringA = "My dog has fleas.";
var stringB = new String("My dog has fleas.");

These two variables might contain the same series of characters but are different data types. The first is a string value, while the second is an instance of a String object. If you place these two values on either side of an equality (= =) operator, JavaScript tries various evaluations of the values to see if there is a coincidence somewhere. In this case, the two variable values would show to be equal, and the following expression:

stringA =  = stringB

returns true.

But the other type of equality operator, the strict equality operator (= = =), performs no data type conversions. Given the variable definitions above, the following expression evaluates to false because the two object types differ, even though their payloads are the same:

stringA =  =  = stringB

If the logic of your code requires you to test for the inequality of two strings, you can use the inequality (!=) and strict inequality (!= =) operators. For example, if you want to process an incorrect entry, the branching flow of your function would be like the following:

if (document.myForm.myTextBox.value.toLowerCase( ) != "new york") {
    // process incorrect entry

The same data type conversion issues apply to the inequality and strict inequality operators as to their opposite partners.

Although the equality and inequality operators go to great lengths to find value matches, you may prefer to assist the process by performing obvious data type conversions in advance of the operators. For instance, if you want to see if an entry to a numeric text box (a string value) is a particular number, you could let the equality operator perform the conversion for you, as in:

if (document.myForm.myTextBox.value =  = someNumericVar) { ... }

Or you could act in advance by converting one of the operands so that both are the same data type:

if (parseInt(document.myForm.myTextBox.value) =  = someNumericVar) { ... }

If you are accustomed to more strongly typed programming languages, you can continue the practice in JavaScript without penalty, while perhaps boosting your script's readability.

1.4.4 See Also

Recipe 2.1 for converting between string and number values; Recipe 3.3 for converting between strings and arrays; Recipe 3.13 for converting a custom object to a string value.

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